From: Liang Chen cl@rock-chips.com
commit 26c7295be0c5e6da3fa45970e9748be983175b1b upstream.
when we create a kthread with ktrhead_create_on_cpu(),the child thread entry is ktread.c:ktrhead() which will be preempted by the parent after call complete(done) while schedule() is not called yet,then the parent will call wait_task_inactive(child) but the child is still on the runqueue, so the parent will schedule_hrtimeout() for 1 jiffy,it will waste a lot of time,especially on startup.
parent child ktrhead_create_on_cpu() wait_fo_completion(&done) -----> ktread.c:ktrhead() |----- complete(done);--wakeup and preempted by parent kthread_bind() <------------| |-> schedule();--dequeue here wait_task_inactive(child) | schedule_hrtimeout(1 jiffy) -|
So we hope the child just wakeup parent but not preempted by parent, and the child is going to call schedule() soon,then the parent will not call schedule_hrtimeout(1 jiffy) as the child is already dequeue.
The same issue for ktrhead_park()&&kthread_parkme(). This patch can save 120ms on rk312x startup with CONFIG_HZ=300.
Signed-off-by: Liang Chen cl@rock-chips.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Steven Rostedt (VMware) rostedt@goodmis.org Link: https://lkml.kernel.org/r/20200306070133.18335-2-cl@rock-chips.com Signed-off-by: Chanho Park chanho61.park@samsung.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/kthread.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)
--- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -199,8 +199,15 @@ static void __kthread_parkme(struct kthr if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags)) break;
+ /* + * Thread is going to call schedule(), do not preempt it, + * or the caller of kthread_park() may spend more time in + * wait_task_inactive(). + */ + preempt_disable(); complete(&self->parked); - schedule(); + schedule_preempt_disabled(); + preempt_enable(); } __set_current_state(TASK_RUNNING); } @@ -245,8 +252,14 @@ static int kthread(void *_create) /* OK, tell user we're spawned, wait for stop or wakeup */ __set_current_state(TASK_UNINTERRUPTIBLE); create->result = current; + /* + * Thread is going to call schedule(), do not preempt it, + * or the creator may spend more time in wait_task_inactive(). + */ + preempt_disable(); complete(done); - schedule(); + schedule_preempt_disabled(); + preempt_enable();
ret = -EINTR; if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {